1. Field of the Invention
The present invention relates to piezoelectric resonators, and more specifically, the present invention relates to piezoelectric resonators, filters and electronic apparatuses utilizing the elastic vibration of piezoelectric layers.
2. Description of the Related Art
Resonant frequencies of piezoelectric thin film resonators that are adapted to utilize the thickness mode vibration of piezoelectric substrates, have an inverse proportional relationship relative to the thickness of the piezoelectric substrates, in very high frequencies. Therefore, piezoelectric substrates included in such piezoelectric resonators must be extremely thin. However, there is a limit to how think the piezoelectric substrates can be. In most piezoelectric resonators of this type, there is a limit of several 100 MHz of high frequencies in practice due to restrictions in the mechanical strength, treatment steps, manufacturing processes and other factors relating to the piezoelectric substrates.
To solve these problems, piezoelectric thin film resonators have been proposed as filters and resonators, etc. The piezoelectric thin film resonator 1 shown in FIG. 1, is manufactured by partially etching a Si substrate 2 using a fine processing method to form a thin film support portion 3, having a thickness of several xcexcm or less, on a portion of the Si substrate 2, and by providing a ZnO piezoelectric thin film 4, having a pair of excitation electrodes 5a and 5b on both sides, on the support portion 3.
The aforementioned piezoelectric thin film resonator 1 has a possibility to extend its high frequency characteristics to as high as several 100 MHz to several 1000 MHz, because the thin film support portion 3 can be made thin, using the fine processing technique, and the piezoelectric thin film 4 can also be made thin by sputtering.
However, a temperature coefficient of resonant frequency (TCF) of ZnO is about xe2x88x9270 ppm/xc2x0 C., and a temperature coefficient of resonant frequency of Si is about xe2x88x9230 ppm/xc2x0 C. Both ZnO and Si have negative temperature coefficients of resonant frequency, and, therefore, a combination of the piezoelectric thin film 4, made of ZnO, and the thin film support portion 3, made of Si, has the disadvantage that temperature characteristics of resonant frequency in the dominant mode become inferior.
In a piezoelectric thin film resonator 6, shown in FIG. 2, an SiO2 thin film 7 is formed on the surface of an Si substrate 2 by thermal oxidation. A thin film support portion 3 is formed from the SiO2 thin film 7 by partially etching the Si substrate 2, and a ZnO piezoelectric thin film 4, having a pair of excitation electrodes 5a and 5b on both sides, is formed on the support portion 3.
A temperature coefficient of resonant frequency of ZnO is about xe2x88x9270 ppm/xc2x0 C., and a temperature coefficient of resonant frequency of SiO2 is about +100 ppm/xc2x0 C. ZnO and SiO2 have temperature coefficients of resonant frequency having opposite signs from each other. By adjusting a ratio of a film thickness of the piezoelectric thin film 4, made of ZnO, to a film thickness of the thin film support portion 3, made of SiO2, at a ratio of about 2:1, it is possible to make the temperature coefficient of resonant frequency, in the dominant mode, small, and to make the temperature characteristics of resonant frequency stable. This is described in Japanese Unexamined Patent Application Publication No. 58-121817.
FIG. 3 is a sectional view illustrating a piezoelectric thin film resonator 9 having another structure. This is the piezoelectric thin film resonator 9, having a floating construction or air bridge construction, manufactured by forming a thin film support portion 12, made of SiO2, on a Si substrate 10 via an air gap 11, and providing a ZnO piezoelectric thin film 13, having excitation electrodes 14a and 14b on both sides, on the support portion 12 that is arranged to be free from the Si substrate 10.
In the piezoelectric thin film resonator 9, similarly to the piezoelectric thin film resonator 6 shown in FIG. 2, by adjusting a ratio of a film thickness of the ZnO piezoelectric thin film to a film thickness of the SiO2 thin film support portion 12 at a proper value, it is possible to make the temperature coefficient of resonant frequency small and to make the temperature characteristics of resonant frequency stable.
In the aforementioned second piezoelectric thin film resonator 6, by a combination of the ZnO piezoelectric thin film 4 and the SiO2 thin film support portion 3, temperature coefficients of resonant frequency can offset each other. In the aforementioned third piezoelectric thin film resonator 9, by a combination of the ZnO piezoelectric thin film 13 and the SiO2 thin film support portion 12, temperature coefficients of resonant frequency can offset each other.
However, ZnO is a piezoelectric, whereas SiO2 is not a piezoelectric. Therefore, in these piezoelectric thin film resonators, resonant responses have been very small and resonant characteristics have been inferior.
In order to overcome and solve the above-described problems, preferred embodiments of the present invention provide piezoelectric resonators having a very stable temperature coefficient of resonant frequency, a very large resonant response, and excellent resonant characteristics.
According to one preferred embodiment of the present invention, a piezoelectric resonator includes a laminated member, at least one pair of electrodes and a substrate. The laminated member includes a piezoelectric laminate body, the piezoelectric laminate body including at least one first piezoelectric layer which has a positive temperature coefficient of a resonant frequency and at least one second piezoelectric layer which has a negative temperature coefficient of a resonant frequency. The pair of electrodes interpose at least one of the first and second piezoelectric layers. The substrate supports the laminated member and holds a support portion of the laminated member such that the support portion vibrates in response to application of a voltage across the pair of electrodes.
According to the structure, by properly adjusting the thickness of each piezoelectric layer, the temperature coefficient of resonant frequency of the entire laminate member becomes nearly zero. Furthermore, because all layers except for electrodes are made of piezoelectric materials, the resonant response of the piezoelectric resonator is excellent, and the resonant characteristics are also excellent. Therefore, piezoelectric resonators, having very stable temperature characteristics, very large resonant responses, and excellent resonant characteristics, are provided.
The laminated structure may also include an insulating layer located between the substrate and the piezoelectric laminate.
According to such a structure, the insulator layer is disposed on the substrate, and because, generally, insulator layers are difficult to be etched with etching liquids used for substrates and layers intended to be etched, the processing in the manufacturing procedures is much easier.
Furthermore, because the insulating layer, and two or more kinds of piezoelectric layers are laminated on the substrate, material parameters of the vibration portion become three or more, and it becomes possible to easily and accurately adjust electromechanical coefficients and piezoelectric characteristics.
Therefore, it is possible to stabilize temperature coefficients of resonant frequency, to greatly increase resonant responses, to achieve excellent resonant characteristics, to greatly simplify the etching process for floating the insulation layer above the substrate, and to greatly increase the design flexibility for other characteristics.
The pair of electrodes may interpose the at least one first piezoelectric layer and the at least one second piezoelectric layer.
According to this unique structure, by applying an electric signal for excitation to the electrodes, all piezoelectric layers can be excited. Therefore, resonant responses of piezoelectric resonators are greatly increased and made very large, and the resultant piezoelectric resonators having very strong and desirable resonant characteristics.
The first piezoelectric layer is preferably made of, as a primary component, one of AlN and PbZrxTi(1xe2x88x92x)O3 (0.54xe2x89xa6xxe2x89xa61), and the second piezoelectric layer is preferably made of, as a primary component, one piezoelectric material selected from the group consisting of ZnO, LiNbO3, LiTaO3, and PbZrxTi(1xe2x88x92x)O3 (0xe2x89xa6xxe2x89xa60.52).
The piezoelectric laminate may include additional first or second piezoelectric layers. In such a case, the additional first or second piezoelectric layers and the at least one first or second piezoelectric layer have substantially the same thickness and interpose the at least second or first piezoelectric layer.
According to the unique structure of this preferred embodiment, the piezoelectric laminate member preferably includes three piezoelectric layers and is symmetric with respect to the center layer. As a result, even if stress is applied in each piezoelectric layer due to temperature change, the stresses are balanced, thereby realizing a piezoelectric resonator that has a very high mechanical strength and does not experience warpage.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof, with reference to the attached drawings.